1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and particularly relates to a semiconductor device with a trench gate and a method of manufacturing the same.
2. Description of Related Art
With a recent technical development, a semiconductor device having a protection feature is put into practical use. For example, Japanese Patent JP-B 3,413,569 (corresponding to U.S. Pat. No. 6,323,518 (B1)) discloses an insulated-gate semiconductor device having a temperature detecting element as an overheat protection feature and a method of manufacturing thereof.
FIG. 1 is a schematic cross-sectional view showing a structure of the semiconductor device disclosed in Japanese Patent JP-B 3,413,569. This insulated-gate semiconductor device includes a trench-type insulated-gate semiconductor element such as a trench-gate MOSFET, and a diode concerning a gate protection for this trench-type insulated-gate semiconductor element. In the trench-type insulated-gate semiconductor element, a plurality of trenches 104 is formed in a main surface of a semiconductor layer 102 on a semiconductor substrate 101. Gate layers 110 connected to a first electrode 113a are formed inside and outside of the plurality of trenches 104 through gate insulating films 105. In a surface opposite to the main surface of the semiconductor layer 102, a second electrode 114 is formed. A diffusion layer 106 connected to a third electrode 107 is formed between the gate layers 110. The gate layer 110 includes a gate layer region on the trenches 104 and a gate layer region extending outside the trenches 104. The first electrode 113a and the gate layer 110 are connected to each other at the gate layer region extending outside the trenches 104. The diode (composed of 121 to 123) is formed on an insulating film 109 formed in the main surface of the semiconductor layer 102 of the semiconductor substrate 101. A film thickness of the diode is less than a film thickness of the gate layer region extending outside the trenches 104 to connect the first electrode 113a with the gate layer 110. That is, in the insulated-gate semiconductor device, a temperature detecting element for overheat protection includes polycrystalline silicon diode (121 to 123) provided on the oxide film 109. The temperature is then detected by means of a forward voltage value of the polycrystalline silicon diode (121 to 123) for the temperature detection.
It is possible to use a bipolar transistor as the temperature detecting element. For example, Japanese Laid-Open Patent Application JP-A 2002-48651 (corresponding to U.S. Pat. No. 6,733,174(B2)) discloses a semiconductor temperature detecting circuit with a bipolar transistor for detecting the temperature. FIG. 2 is a circuit diagram showing a structure of the temperature detecting element (the temperature sensor) of the semiconductor temperature detecting circuit disclosed in Japanese Laid-Open Patent Application JP-A 2002-48651. This temperature detecting element is represented by the circuit diagram using a three-stage Darlington connection, which includes NPN bipolar transistors Tr1 to Tr3.
As a related technique, Japanese Laid-Open Patent Application JP-A-Heisei 06-326320 discloses a semiconductor device and a method for manufacturing thereof. The semiconductor device includes a first-conduction-type semiconductor substrate, a first-conduction-type semiconductor layer, a first-conduction-type first region, a first-conduction-type second region, a second-conduction-type base region of a power semiconductor device, a first-conduction-type source region of the power semiconductor device, a gate trench, a gate oxide film of the power semiconductor device, a gate electrode of the power semiconductor device, a second-conduction-type buried isolation layer, and an element-isolation trench. The first-conduction-type semiconductor layer is formed on the first-conduction-type semiconductor substrate. The first-conduction-type first region is formed in the first-conduction-type semiconductor layer and includes the power semiconductor element. The first-conduction-type second region is formed in the first-conduction-type semiconductor layer and includes a control circuit element. The second-conduction-type base region of the power semiconductor device is formed in a surface region of the first region. The first-conduction-type source region of the power semiconductor element is formed in the surface region of the first region and surrounded by the second-conduction-type base region. The gate trench is formed in the first-conduction-type source region, and extends from the main surface of the first-conduction-type semiconductor layer thereinto to pass through the second-conduction-type base region. The gate oxide film of the power semiconductor device is formed on a side wall of the gate trench. The gate electrode of the power semiconductor element is formed in the gate trench and placed on the gate oxide film. The second-conduction-type buried isolation layer is formed in the second region, or between the first-conduction-type semiconductor substrate and the second region. The element-isolation trench is formed at least between the first region and the second region, and extends from the main surface of the first-conduction-type semiconductor layer to the second-conduction-type buried isolation layer. Both the second-conduction-type buried isolation layer and the element-isolation trench isolate the second region of the first-conduction-type semiconductor layer from the other regions of the first-conduction-type semiconductor layer.
However, the inventor's recent research has revealed this time, that the temperature detecting element formed in the semiconductor substrate has the problems as follows.
FIG. 1 shows only a single silicon diode for the temperature detection. Typically, however, a plurality of diodes connected in series to one another is used for the temperature detecting element in order to increase the temperature coefficient of the temperature detecting element. As shown in this example, in the case where the silicon diode for the temperature detection is the polycrystalline silicon diode, the oxide film 109 having a poor thermal conductivity lies between the diode and an MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) that is a trench-type insulated-gate semiconductor element. In this case, the thermal conduction to the diode is prone to be poor with regard to the heat generated at a MOSFET side in a short period of time. This increases the difference in temperature between the MOSFET and the diode, and thus an error can easily occur in the temperature detection. This leads to a shortcoming in which it becomes difficult to protect the MOSFET more adequately.
A diode or a bipolar transistor using a diffusion layer can be contemplated as a temperature detecting element that can be formed without interposing the oxide film. In the case of using such an element having the diffusion layer (the device formed on the surface of the semiconductor substrate), however, a field plate is required to be provided on the peripheral diffusion layer to increase a withstand voltage of the peripheral diffusion layer so that the withstand voltage at each of the peripheral junction portions of the temperature detecting element do not decrease (the leakage current do not increase at each of the peripheral junction portions). This is because, if mobile ions penetrate into a chip surface to make the surface withstand voltage of the peripheral junction portion lower, the characteristic of the temperature detecting element is varied, thereby being unable to implement the protection more adequately and precisely.
In order to implement the protection more adequately and precisely, the inventor has considered that a plurality of stages of bipolar transistors can be used for a temperature detecting element. Because the inventor's recent research has found that an semiconductor device, which is represented by the circuit diagram using the Darlington connection as shown in FIG. 2, possesses characteristics as the temperature detecting element, shown in FIGS. 3 and 4. FIG. 3 is a graph showing a voltage-current characteristic of the temperature detecting element. FIG. 4 is a graph showing the temperature dependence of the voltage of the temperature detecting element when the current is 10 μA. As shown in these figures, it has found that this temperature detecting element possesses the characteristics similar to those of the temperature detecting element with the diodes in three-stage series connection.
When the plurality of stages of bipolar transistors is used, for example, as shown in FIG. 2, a field plate is required for each bipolar transistor in order to implement the protection more adequately and precisely. However, when the field plate is provided for each temperature detecting element as mentioned above, a problem arises that a layout area of the temperature detecting element (the protected device) becomes larger.